Studies were undertaken to measure specific cytotoxic T-cell responses and humoral antibody responses by B-cells in vitro in normal individuals and patients with immunologic deficiency states. Patients were heterogeneous with regard to their ability to generate cytotoxic T-cells in vitro. Most patients with hypogammaglobulinemia produced cytotoxic T-cells normally, while patients with the Wiskott-Aldrich syndrome and ataxia-telangiectasia produced almost no cytotoxic T-cells. Cells from normal individuals produce specific antibody in vitro when stimulated with antigen. This was also true for 5 of 11 patients with hypogammaglobulinemia who produced no antibody in vivo. Thus a subset of hypogammaglobulinemic individuals have an immune deficiency which can be corrected in vitro. In contrast none of 7 ataxia-telangiectasia patients and none of 5 Wiskott-Aldrich syndrome patients produced antibody in vitro. Defects in the production of cytotoxic T-cells and specific antibodies may contribute to the increased incidence of neoplasia observed in immunodeficiency diseases.